Calculating sealed box resonance (f3)

Ok so I'm still trying to design this 4th order but not before I completely understand everything first.
I found somewhere that to find the F3 of the sealed chamber (or any plain ol sealed box) is used by a simple formula....

[(vas/vb)+1)^2] x Fs .... Assuming both vas and vb are in liters, I should with a certain vb shoot for an F3 of 45 hz correct? Or could I shoot for a lower F3 in the sealed chamber since I want to tune my ported chamber @ 40hz to pound lows??
The problem I've been having is I do this equation with the woofers I plan on using (American bass TNT) and I keep getting an F3 that's EXTREMELY high.